The invention is based on a priority application EP 02360333.5 which is hereby incorporated by reference.
The invention relates to a planar lightwave circuit (PLC) comprising an optical device.
Filtering or duplexing functions are needed in many parts of an optical network, p.ex. as multiplexers, demultiplexers, in optical amplifiers, and as band filters. These functions can be realized using thin film filters or mirrors. To this end it is desirable to integrate a thin film filter within a planar waveguide.
It is known to insert a small piece of a multilayer thin film filter deposited on a bulk glass substrate with narrow tolerances into a slit in a planar optical waveguide (filter on glass approach). This approach has to compromise between the use of a thick glass substrate for the inserted thin film filter, which is mechanically stable but leads to increased insertion loss due to non-guided coupling between the waveguides of the planar lightwave circuit and the use of a thin substrate for the thin film filter. The thickness of the thin substrate is in the order of a few micrometers. This limits the design of a multilayer thin film filter, e.g. with respect to athermalization. Temperature variations lead to bowing of the substrate, resulting in a change of the filter characteristics. Furthermore, thin substrates are difficult to handle and to attach in a narrow slit of the planar lightwave circuit.
Alternatively it has been suggested to deposit a filter layer stack upon a polished edge of a planar lightwave circuit. This, however, can only be used in cases where the filter or mirror can be placed at the edge of the die layout. Furthermore, the polishing and coating processes work only for single pieces or small batches, leading to high costs of fabrication.
The use of lattice-type filters such as Mach-Zehnder structures or arrayed waveguide gratings has also been suggested. However, lattice-type filters with sharp filter characteristics are generally large compared with thin film filters having comparable performance. In addition, athermalization of lattice-type filters is difficult to achieve.